This only occurs when using Symfony stack, with legacy fallback.

In a legacy module, when you set a session variable and then perform a redirection (e.g. using $Module->redirectTo( '/' )) to a page managed by Symfony stack, created session will be lost during the redirection process.

Steps to reproduce

1. Create a legacy module with the following code:

   <?php
   $Module = $Params['Module'];
   $Result = array();
   
   $http = eZHTTPTool::instance();
   $http->setSessionVariable( 'foo', time() );
   $Module = $Params['Module'];
   return $Module->redirectTo( "/my_custom_controller_uri" );
   

2. Create a custom symfony controller with a /my_custom_controller_uri route, and ensure you dump foo session variable:

   var_dump( $this->getRequest()->getSession()->get( 'foo' ) );
   

3. Access to your legacy module URL, you'll be redirected to your custom controller and foo session variable will be null